In coding of sound signals such as speech and music, a method of performing the coding by using linear prediction coefficients obtained by performing linear prediction analysis on an input sound signal is widely used.
In order to make it possible to obtain, on the part of a decoding device, the information on the linear prediction coefficients used in coding processing by decoding, a coding device codes the linear prediction coefficients and sends a code corresponding to the linear prediction coefficients to the decoding device. In Non-patent Literature 1, a coding device converts linear prediction coefficients into a sequence of LSP (Line Spectrum Pair) parameters which are parameters in a frequency domain and equivalent to the linear prediction coefficients and sends an LSP code obtained by coding the sequence of LSP parameters to a decoding device.
In Non-patent Literature 1, in order to reduce the code amount of the LSP code, a vector coding and decoding technology using moving average prediction (MA prediction) is used.
First, the flow of coding processing will be described.
<Linear Prediction Coefficient Coding Device 80>
FIG. 1 depicts the configuration of an existing linear prediction coefficient coding device 80.
To the linear prediction coefficient coding device 80, LSP (Line Spectrum Pairs) parameters θf[1], θf[2], . . . , θf[p] of each frame are input, and the linear prediction coefficient coding device 80 performs the following processing of a predictive subtraction unit 83, a vector coding unit 84, and a delay input unit 87 on a frame-by-frame basis, obtains an LSP code Cf, and outputs the LSP code Cf. Incidentally, f represents a frame number and p represents a prediction order.
When an input sound signal Xf is input to the linear prediction coefficient coding device 80, the linear prediction coefficient coding device 80 is also provided with a linear prediction analysis unit 81 and an LSP calculation unit 82, and the frame-by-frame input sound signals Xf are consecutively input thereto and the following processing is performed on a frame-by-frame basis.
Hereinafter, specific processing of each unit will be described.
<Linear Prediction Analysis Unit 81>
The linear prediction analysis unit 81 receives the input sound signal Xf, performs linear prediction analysis on the input sound signal Xf, obtains linear prediction coefficients af[1], af[2], . . . , af[p], and outputs the linear prediction coefficients af[1], af[2], . . . , af[p]. Here, af[i] represents an ith-order linear prediction coefficient that is obtained by performing linear prediction analysis on an input sound signal Xf of an fth frame.
<LSP Calculation Unit 82>
The LSP calculation unit 82 receives the linear prediction coefficients af[1], af[2], . . . , af[p], obtains LSP parameters θf[1], θf[2], . . . , θf[p] from the linear prediction coefficients af[1], af[2], . . . , af[p], and outputs an LSP parameter vector Θf=(θf[1], θf[2], . . . , θf[p])T that is a vector using the obtained LSP parameters as elements thereof. Here, θf[i] is an ith-order LSP parameter corresponding to the input sound signal Xf of the fth frame.
<Predictive Subtraction Unit 83>
The predictive subtraction unit 83 is formed of, for example, a storage 83c storing a predetermined coefficient α, a storage 83d storing a predictive mean vector V, a multiplication unit 88, and subtraction units 83a and 83b. 
The predictive subtraction unit 83 receives the LSP parameter vector Θf and a preceding-frame quantization differential vector {circumflex over ( )}Sf−1.
The predictive subtraction unit 83 generates a differential vector Sf=Θf−V−α×{circumflex over ( )}Sf−1=(sf[1], sf[2], . . . , sf[p])T that is a vector obtained by subtracting the predictive mean vector V and a vector α{circumflex over ( )}Sf−1 from the LSP parameter vector Θf and outputs the differential vector Sf.
Incidentally, the predictive mean vector V=(v[1], v[2], . . . , v[p])T is a predetermined vector stored in the storage 83d and simply has to be obtained in advance from, for example, a sound signal for learning. For example, in the linear prediction coefficient coding device 80, by using a sound signal picked up in the same environment (for instance, the same speaker, sound pick-up device, and place) as the sound signal to be coded as an input sound signal for learning, LSP parameter vectors of many frames are obtained, and the average thereof is used as the predictive mean vector.
The multiplication unit 88 obtains a vector α×{circumflex over ( )}Sf−1 by multiplying a decoded differential vector {circumflex over ( )}Sf−1 of a preceding frame by the predetermined coefficient α stored in the storage 83c. 
Incidentally, in FIG. 1, by using the two subtraction units 83a and 83b, first, after the predictive mean vector V stored in the storage 83d is subtracted from the LSP parameter vector Θf in the subtraction unit 83a, the vector α×{circumflex over ( )}Sf−1 is subtracted in the subtraction unit 83b, but the above may be performed the other way around. Alternatively, the differential vector Sf may be generated by subtracting, from the LSP parameter vector Θf, a vector V+α×{circumflex over ( )}Sf−1 obtained by adding the predictive mean vector V and the vector α×{circumflex over ( )}Sf−1.
The differential vector Sf of the present frame may also be called a vector that is obtained by subtracting a vector containing at least a prediction based on a past frame from a vector (an LSP parameter vector Θf) based on coefficients which are convertible into linear prediction coefficients of more than one order of the present frame.
<Vector Coding Unit 84>
The vector coding unit 84 receives the differential vector Sf, codes the differential vector Sf, and obtains an LSP code Cf and a quantization differential vector {circumflex over ( )}Sf=({circumflex over ( )}sf[1], {circumflex over ( )}sf[2], . . . , {circumflex over ( )}sf[p])T corresponding to the LSP code Cf and outputs the LSP code Cf and the quantization differential vector {circumflex over ( )}Sf. For coding of the differential vector Sf, any one of the well-known coding methods may be used, such as a method of vector quantizing the differential vector Sf, a method of dividing the differential vector Sf into a plurality of subvectors and vector quantizing each of the subvectors, a method of multistage vector quantizing the differential vector Sf or the subvectors, a method of scalar quantizing the elements of a vector, and a method obtained by combining these methods.
Here, an example of a case in which the method of vector quantizing the differential vector Sf is used will be described.
The vector coding unit 84 searches for a candidate differential vector closest to the differential vector Sf from a plurality of candidate differential vectors stored in a vector codebook 86 and outputs the candidate differential vector as the quantization differential vector {circumflex over ( )}Sf, and outputs a differential vector code corresponding to the quantization differential vector {circumflex over ( )}Sf as the LSP code Cf. Incidentally, the quantization differential vector {circumflex over ( )}Sf corresponds to a decoded differential vector which will be described later.
<Vector Codebook 86>
In the vector codebook 86, candidate differential vectors and differential vector codes corresponding to the candidate differential vectors are stored in advance.
<Delay Input Unit 87>
The delay input unit 87 receives the quantization differential vector {circumflex over ( )}Sf, holds the quantization differential vector {circumflex over ( )}Sf, delays the quantization differential vector {circumflex over ( )}Sf by one frame, and outputs the resultant vector as a preceding-frame quantization differential vector {circumflex over ( )}Sf−1. That is, if the predictive subtraction unit 83 has performed processing on a quantization differential vector {circumflex over ( )}Sf of an fth frame, the delay input unit 87 outputs a quantization differential vector {circumflex over ( )}Sf−1 on an f−1th frame.
<Linear Prediction Coefficient Decoding Device 90>
FIG. 2 depicts the configuration of an existing linear prediction coefficient decoding device 90. To the linear prediction coefficient decoding device 90, frame-by-frame LSP codes Cf are consecutively input, and the linear prediction coefficient decoding device 90 obtains a decoded predictive LSP parameter vector {circumflex over ( )}Θf=({circumflex over ( )}θf[1], {circumflex over ( )}θf[2], . . . , {circumflex over ( )}θf[p]) by decoding the LSP code Cf on a frame-by-frame basis.
Hereinafter, specific processing of each unit will be described.
<Vector Decoding Unit 91>
A vector decoding unit 91 receives the LSP code Cf, decodes the LSP code Cf, obtains a decoded differential vector {circumflex over ( )}Sf corresponding to the LSP code Cf, and outputs the decoded differential vector {circumflex over ( )}Sf. For decoding of the LSP code Cf, a decoding method corresponding to the coding method adopted by the vector coding unit 84 of the coding device is used.
Here, an example of a case in which a decoding method corresponding to the method adopted by the vector coding unit 84, the method of vector quantizing the differential vector Sf, is used will be described.
The vector decoding unit 91 searches for a plurality of differential vector codes corresponding to the LSP code Cf from differential vector codes stored in a vector codebook 92 and outputs a candidate differential vector corresponding to the differential vector codes as the decoded differential vector {circumflex over ( )}Sf. Incidentally, the decoded differential vector {circumflex over ( )}Sf corresponds to the above-described quantization differential vector {circumflex over ( )}Sf and corresponding elements take the same values if there are no transmission errors and no errors and the like in the course of coding and decoding.
<Vector Codebook 92>
In the vector codebook 92, the candidate differential vectors and the differential vector codes corresponding to the candidate differential vectors are stored in advance. Incidentally, the vector codebook 92 shares information in common with the vector codebook 86 of the above-described linear prediction coefficient coding device 80.
<Delay Input Unit 93>
A delay input unit 93 receives the decoded differential vector {circumflex over ( )}Sf, holds the decoded differential vector {circumflex over ( )}Sf, delays the decoded differential vector {circumflex over ( )}Sf by one frame, and outputs the resultant vector as a preceding-frame decoded differential vector {circumflex over ( )}Sf−1. That is, if a predictive addition unit 95 performs processing on a decoded differential vector {circumflex over ( )}Sf of an fth frame, the delay input unit 93 outputs a decoded differential vector {circumflex over ( )}Sf−1 of an f−1th frame.
<Predictive Addition Unit 95>
A predictive addition unit 95 is formed of, for example, a storage 95c storing a predetermined coefficient α, a storage 95d storing a predictive mean vector V, a multiplication unit 94, and addition units 95a and 95b. 
The predictive addition unit 95 receives the decoded differential vector {circumflex over ( )}Sf of the present frame and the preceding-frame decoded differential vector {circumflex over ( )}Sf−1.
The predictive addition unit 95 generates a decoded predictive LSP parameter vector {circumflex over ( )}Θf. (={circumflex over ( )}Sf+V+α{circumflex over ( )}Sf−1) that is a vector obtained by adding the decoded differential vector {circumflex over ( )}Sf, the predictive mean vector V=(v[1], v[2], . . . , v[N])T, and a vector α×{circumflex over ( )}Sf−1 and outputs the decoded predictive LSP parameter vector {circumflex over ( )}Θf.
The multiplication unit 94 obtains the vector α×{circumflex over ( )}Sf−1 by multiplying the preceding-frame decoded differential vector {circumflex over ( )}Sf−1 by the predetermined coefficient α stored in the storage 95c. 
In FIG. 2, by using the two addition units 95a and 95b, first, after the vector α×{circumflex over ( )}Sf−1 is added to the decoded differential vector {circumflex over ( )}Sf of the present frame in the addition unit 95a, the predictive mean vector V is added in the addition unit 95b, but the above may be performed the other way around. Alternatively, the decoded predictive LSP parameter vector {circumflex over ( )}Θf may be generated by adding a vector obtained by adding the vector α×{circumflex over ( )}Sf−1 and the predictive mean vector V to the decoded differential vector {circumflex over ( )}Sf.
Incidentally, it is assumed that the predictive mean vector V used here is the same as the predictive mean vector V used in the predictive subtraction unit 83 of the above-described linear prediction coefficient coding device 80.
<Decoded Predictive Linear Prediction Coefficient Calculation Unit 96>
If linear prediction coefficients are necessary, the linear prediction coefficient decoding device 90 may be provided with a decoded predictive linear prediction coefficient calculation unit 96. In this case, the decoded predictive linear prediction coefficient calculation unit 96 receives the decoded predictive LSP parameter vector {circumflex over ( )}Θf, converts the decoded predictive LSP parameter vector {circumflex over ( )}Θf into decoded predictive linear prediction coefficients {circumflex over ( )}af[1], {circumflex over ( )}af[2], . . . , {circumflex over ( )}af[p], and outputs the decoded predictive linear prediction coefficients {circumflex over ( )}af[1], {circumflex over ( )}af[2], . . . , {circumflex over ( )}af[p].